The hepatitis delta virus (Delta) is a defective RNA pathogen dependent for replication on obligatory helper functions provided by the hepatitis B virus (HBV). The virion is a particle of 35-37 nm, which encloses within a HBsAg coat the RNA genome and the Delta antigen, the specific immunologic expression of the new pathogen. Infection is diagnosed by the presence of Delta antigen in liver and the finding of antibody to Delta in serum. The virus is infectious for primates but can also adapt to non-primates. Distribution is world-wide with peak prevalence in the Mediterranean basin, the Middle East and areas of Africa. Transmission occurs by direct parenteral inoculation and by non-parenteral routes linked to close body contacts. Delta is highly pathogenic and a cause or an aggravating factor of HBsAg-positive liver disease. Infection superimposed on acute HBV hepatitis may induce a fulminant illness. Superinfection of carriers of HBsAg may convert a previously asymptomatic carrier in a carrier with chronic hepatitis, or accelerate the downward clinical course of chronic underlying HBV disease. Carriers of HBsAg probably represent the reservoir of the virus.